The present invention relates to parts made of ceramic matrix composite material. The invention relates more particularly to improving the surface state of such parts.
In aeroengines, and in particular in the gas turbines of such engines, parts that present aerodynamic shapes, such as blades, are conventionally made out of metal alloys using a casting method and local machining. Present and future requirements for aeroengines in terms of reducing specific energy consumption, reducing pollution, etc., are giving rise to a significant increase in the weight of such engines, in particular in the low pressure stages of their turbines.
The blades constitute a large part of the weight of low pressure stages. In order to reduce the weight significantly, while also accepting operating temperatures higher than those made possible with present metal alloys, one solution would be to make the blades out of ceramic matrix composite materials.
Ceramic matrix composite (CMC) materials are examples of so-called “thermostructural” composite materials, i.e. composite materials having good mechanical properties and the ability to conserve their properties at high temperature. In addition, such parts, such as blades, when made out of CMC present a significant saving in weight compared with the same parts made using the usual metal alloys.
In well-known manner, CMC parts are formed of fiber reinforcement made of refractory fibers (carbon fibers or ceramic fibers) and densified with a ceramic matrix, in particular a refractory carbide, nitride, oxide, . . . matrix. Typical examples of CMC materials are C—SiC materials (carbon fiber reinforcement and silicon carbide matrix), SiC—SiC materials, and C—C/SiC materials (matrix both of carbon and of silicon carbide). The fabrication of parts made out of CMC material is well known. The fiber reinforcement may be densified using a liquid technique (impregnation with a resin that is a precursor of the ceramic matrix and transformation of the resin into ceramic by curing and pyrolysis, which process may be repeated) or by using a gas technique (chemical vapor infiltration).
Nevertheless, CMC parts present a surface appearance that is undulating and relatively rough, which can be incompatible with the aerodynamic performance required of parts such as blades. The surface undulation is due to the fiber reinforcement, whereas the roughness is associated with the “seal-coat” ceramic matrix, in particular when the matrix is deposited by chemical vapor infiltration (CVI).
Conversely, parts made out of metal alloys and by the associated methods present a surface appearance that is smooth with very little roughness (of the order of 1 micrometers (μm)).
One solution for improving the surface state of a CMC part consists in applying a liquid composition to its surface, the liquid containing a ceramic precursor polymer, e.g. silicon carbide, and a refractory solid filler in the form of grains enabling a ceramic coating to be formed. The ceramic coating serves to smooth out the undulations present in the surface of the part. This step is followed by depositing ceramic, e.g. SiC, made using chemical vapor infiltration (CVI) for a duration of about 30 hours, thereby serving to bond together the grains of refractory filler. Such a method of treating the surface of a CMC part is described in Document US 2006/0141154.
Although that method makes it possible to improve significantly the surface state of a CMC part by reducing its undulations to 40 μm and its surface roughness to values lying in the range 2 μm to 5 μm (conditioned by the CVI deposition of the ceramic), the need for additional CVI after the ceramic coating has been formed leads to a significant increase in the cost and in the length of time required for fabricating the part.
Consequently, there exists a need for a coating for smoothing the surface of a CMC part that is less penalizing in terms of the duration and the cost of fabricating the part. Such a smoothing coating may be formed by a vitreous coating that is deposited on the part.
Nevertheless, the vitreous smoothing coating needs to satisfy various conditions in order to be adapted to the structural and functional characteristics of CMC parts. The vitreous smoothing coating must in particular present, at least while it is being applied on the part, surface tension and possibly also viscosity that are adapted to smoothing, i.e. that make it possible for the coating to be spread easily and uniformly over the surface of the part. The smoothing coating must also have a coefficient of thermal expansion that is close to that of the CMC material of the part in order to avoid differential expansion in the part when it is exposed to high temperatures. Finally, the coating used must also present a melting temperature that is higher than the utilization temperature of the CMC part so as to guarantee the integrity of the coating at said temperature, which may be as high as 1100° C. for the blades of gas turbines, for example.